Effects of annealing on thermochromic properties of W-doped vanadium dioxide thin films deposited by electron beam evaporation

Thermochromic vanadium dioxide (VO2) undergoes a fully reversible semiconductor-metal transition (SMT) at a critical temperature (Tt) of ~68°C with a dramatic change in electric and optical properties, which makes it an attractive candidate for its application in smart windows. Switchable VO2 and W-doped vanadium dioxide (WxV1−xO2) thin films are grown over quartz substrates via electron beam evaporation technique by using VO2/WxV1−xO2 as targets at room temperature (RT) followed by post annealing process at different temperatures. The as-deposited films are amorphous, and that transform to monoclinic VO2 (VO2(M)) with (011)-preferred orientation after annealing at 500°C under vacuum. The (011) peak of W-doped VO2 films shifts to a lower diffraction angle as compared with un-doped VO2 films which confirms the incorporation of W ions into the VO2 lattice. Temperature dependent optical transmittance measurement demonstrates the thermochromic properties, with a reduction in the phase transition temperature (Tt) as observed in W-doped VO2 films, which is attributed to the variation of electron structure in VO2 due to doping. The films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and optical transmittance measurement.